This invention involves polymer-bound ultraviolet absorbing stabilizers. More particularly, it involves the use of such stabilizers in coating compositions.
Many types of organic compositions, including coating, fibers, films, moldings and dyes, are sensitive to and degraded by ultraviolet radiation. The usual source of such radiation is sunlight, although fluorescent and arc lamps also provide more or less.
Various means are used to minimize the deleterious effects of such degradation. Ultraviolet absorbing stabilizers absorb the ultraviolet radiation and ultimately convert it largely to heat at low enough levels that the heat is much less deleterious than the radiation. Some may also be converted to less deleterious types of radiation such as visible light. Ultraviolet sensitizers also absorb ultraviolet radiation but use it generally to generate free radicals which can be useful in initiating cross linking in specifically tailored systems but which are normally thought of as degradative or deleterious in most organic compositions. Radical quenchers serve as traps for free radicals generated by ultraviolet degradation in a degradative process and thus tend to slow or stop the propagation of undesired effects. Antioxidants retard oxidation of organic compositions and thereby minimize the secondary effect of oxidation by which ultraviolet degradation does much of its damage.
Ultraviolet absorbing stabilizers and sensitizers can both be said to be ultraviolet absorbers, but the former are good for preserving the integrity of an organic composition while the latter promote change. Free radical quenchers and antioxidants operate at a secondary level to minimize damage from free radicals and other effects caused by ultraviolet radiation, heat, ozone or other causes.
Ultraviolet absorbing stabilizers are generally monomeric, although they are sometimes provided as oligomers or with bound groups which increase the molecular weight. Increased molecular weight and the presence of reactive groups can minimize the problem caused by mobility of the unmodified stabilizer. As used in a coating, film or other organic body, an unmodified stabilizer may tend to exude from the body and bloom on the surface or even evaporate at an undesirable rate.
The use of dihydroxy benzophenone bound into an acrylic copolymer through glycidyl methacrylate and optionally blended with other polymers as a coating composition is disclosed in U.S. Pat. Nos. 3,328,491--Fertig, et al; 3,340,231--Fertig, et al; 3,341,493--Goldberg, et al; (all 1967); 3,288,880--Tocker (1966); 3,365,421--Horton, et al (1968); and 3,445,566--Skoultchi, et al (1969). Advantages mentioned include decreased migration of the ultraviolet stabilizer and improved compatability and stability. The dihydroxy benzophenone or a derivative of it is reacted first with glycidyl methacrylate or a derivative of it, and this reaction product is either homopolymerized or copolymerized with other acrylic monomers. Other polymers taught for blending into the paint include polystyrene, polypropylene, polyvinyl fluoride and chloride, vinyl chloride/vinylidene chloride copolymers, polymethyl methacrylate, isoprene, vinyl acetate and cellulosics.
Urethane polymers have been light-stabilized by incorporating into the prepolymer solution 4-amino- or 4-(3-aminopropyl)amino-2,2,6,6-tetramethylpiperidine, as disclosed in German Offenlegungsschrift Nos. 2,642,374--Pfahler, et al (1979) and 2,642,386--Oertel, et al (1976), or bis(2,2,6,6-tetramethyl-4-piperidinyl)sebacate, as disclosed in German Offenlegungsschrift No. 2,820,419--Thomas (1978).
The effectiveness of 2,2,6,6-tetraalkylpiperidines as ultraviolet light stabilizers also is evident in the art as disclosed in German Offenlegungsschrift No. 2,748,362--Oertel, et al (1978) and by F. E. Karrer, Makromol. Chem 181, 595-633 (1980), where in such disclosures the stabilizer is incorporated into a free radical polymerizable monomer. The stabilizer-containing monomer, for example, 2,2,6,6-tetramethyl-4-piperidyl acrylate can be homopolymerized or copolymerized with appropriate free radical copolymerizable monomers to provide polymers having improved light stability. Similar disclosures of acrylate and methacrylate monomers which contain heterocyclic groups and which are polymerizable to polymers exhibiting light stability are made in U.S. Pat. Nos. 4,210,612 (1980) and 4,294,949 (1981)-- both to Karrer.
However, such ultraviolet non-absorbing stabilizers as piperidine derivatives, whether polymer-bound or not, generally are highly polar and may fail to give water resistance to coatings. Furthermore, since they act on the indirect effects of ultraviolet degradation, they can only be effective throughout the bulk thickness of a coating rather than being able to concentrate their effects at the surface as can absorbing stabilizers.
U.S. Pat. No. 4,308,362-- Wiezer, et al (1981) discloses copolymers having repeat units of maleic or fumaric acid esters or amide, the ester or amide groups of which include alkylpiperidine moieties. The alkylpiperidine moieties impart light stability to the polymers. The light stabilizing portion of the polymer is characterized by low volatility, good migration stability and good compatibility with the substrate polymer to which the light stability characteristic is to be imparted. Azo and peroxy free radical initiators containing ultraviolet light stabilizing groups and their use in preparing free radical initiated polymers which exhibit ultraviolet light stability are disclosed in U.S. Pat. No. 3,956,269--Sheppard, et al (1976). Numerous types of ultraviolet light stabilizing compounds, incorporated into the free radical initiator, are disclosed, for example, phenyl salicylates, o-hydroxybenzophenones, cyanoacrylates, benztriazoles and resorcinol monobenzoate. One problem with incorporating ultraviolet absorbing stabilizers in a polymer by bonding them to the polymerization initiator such as azo and peroxy initiators is that this approach inherently limits the percentage of stabilizers that can be incorporated at usefully high molecular weights. Sheppard, et al do not mention higher levels than 10% by weight of stabilizer in a coating, and they do not use more than 2 mers of stabilizer in a polymer molecule.
U.S. Pat. No. 3,429,852--Skoultchi (1969) teaches ethylenically-unsaturated benzophenone sensitizer acrylic copolymers which can be used to provide cross linking in a paint film.
U.S. Pat. No. 2,970,066--Brasure (1961) discloses urethane polymers with ultraviolet stabilizers including dihydroxy benzophenone and aryl homologs of methyl beta-resorcylate, including reacting the stabilizer with an isocyanate prepolymer, and perhaps leaving some reactive isocyanate groups in the reacted polymer.
U.S. Pat. No. 3,441,545--Blatz, et al (1969) discloses reacting ultraviolet stabilizers including dihydroxy benzophenone and stabilizer precursors including resorcinol monobenzoate with various prepolymers through an acid halide group. While useful for various purposes including some types of coatings, residual halides could cause difficulties in some end uses.
U.S. Pat. Nos. 4,310,650 and 4,362,895, both to Gupta, et al (1982), disclose acrylic-benzophenone stabilizer copolymers with residual double bonds, having no crosslinkers and a maximum of 5% stabilizer, and being copolymerized in a single step. (Parts, percentages and proportions herein are by weight except where indicated otherwise.)
Also, Chang's U.S. Pat. Nos. 4,208,465 (1980) and 4,355,071 (1982) teach that 1-20% of ultraviolet stabilizers can be used, based on paint solids, and his European Patent Publication No. 29595 (1981) teaches glycidyl acrylic polymers with melamine cross linkers and with ultraviolet stabilizers. However, his stabilizers are not polymer bound.
The above-mentioned patents are incorporated herein by reference.
The usual approach to ultraviolet stabilization is to use strongly absorbent stabilizers in low concentration such as 0.5-3% of the total composition such as hydroxy phenyl benztriazoles including Tinuvin 328 sold by Ciba Geigy. Higher concentrations have been generally thought to be uneconomical because of the high cost of the stabilizer and unstable because of mobility.
However, the present invention involves the discovery that certain types of ultraviolet absorbing stabilizers which need not have outstanding efficiency can be used in relatively high concentrations when chemically attached to a polymeric binder as a substantial part of the composition being protected.
This permits making a thin paint layer or a thin film which is quite opaque to ultraviolet radiation but which does effectively transmit visible light. This concept can be used advantageously in many applications, such as: an outer clear layer of paint over other paint layers, some of which may be pigmented; a coating on fibers or fabrics which may contain ultraviolet sensitive dyes or other constituents, coatings or films for glass to minimize ultraviolet penetration through the glass; coatings for wood or plastics; coatings, films or bulk additives in molding resins used as refractors or housing for fluorescent or arc lamps such as high pressure mercury arc lamps; hair spray to minimize sun bleaching or discoloration of hair colorants, as in U.S. Pat. No. 3,445,566--Skoultchi (1969); coatings for color photographs; and other potential applications.
The present invention seems particularly useful for a thin unpigmented clear coat to be used as the top layer of an automobile finish system which may have one or more pigmented layers over a primer, topped by one or more clear layers to enhance the aesthetics of the car, giving a higher gloss.
The present invention can provide an outer clear coat in an automotive color coat/clear coat finish system that will give strong enough ultraviolet stabilization to localize the degradation in the outer regions of the surface. Instead of intercoat adhesion failure and flaking off of the outer layers, as sometimes occurs with color coat/clear coat finishes, this can change the mode of aging to slow, minor chalking of the surface. Gloss can remain high with the refractive indices of the constituents chosen properly, and polishing can repair the effects of this type of aging.